The present invention relates to insulated metal-based powder particles that can be annealed to temperatures of 480xc2x0 C. or higher. The present invention also relates to methods of making the annealable insulated metal-based powder particles and methods of making core components from the insulated metal-based powder particles. The core components produced therefrom are particularly useful for low frequency alternating current applications.
Insulated metal-based powders have previously been used to prepare core components. Such core components are used, for example, in electrical/magnetic energy conversion devices such as generators and transformers. Important characteristics of core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetized, or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetizing force or field intensity. Core loss, which is an energy loss, occurs when a magnetic material is exposed to a rapidly varying field. The core losses are commonly divided into two categories: hysteresis and eddy-current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the metal-based core component. The eddy-current loss is brought about by the production of electric currents in the metal based core component due to the changing flux caused by alternating current (AC) conditions.
One consideration in the manufacture of core components from powder materials is that the insulated metal powder needs to be suited for molding. For example, it is desirable for the insulated metal powder to be easily molded into a high density component, having a high pressed strength. These characteristics also improve the magnetic performance of the magnetic core component. It is also desirable that the core component so formed be easily ejected from the molding equipment.
Various insulating materials have been tested as coatings for metal-based powder particles. For example, U.S. Pat. No. 3,933,536 to Doser et al. discloses epoxy-type systems, and magnetic particles coated with resin binders; and U.S. Pat. No. 3,935,340 to Yamaguchi et al. discloses plastic-coated metal powders for use in forming conductive plastic-molded articles and pressed powder magnetic cores. U.S. Pat. No. 5,198,137 to Rutz et al., discloses an iron powder composition where the iron powder is coated with a thermoplastic material and admixed with boron nitride powder. The boron nitride reduces the stripping and sliding die injection pressures during molding at elevated temperatures and also improves magnetic permeability.
A further improvement in insulated metal-based powder particles has been the development of xe2x80x9cdoubly coated metal-based powder particles.xe2x80x9d For example, U.S. Pat. No. 4,601,765, to Soileau et al. discloses iron particles that are first coated with an inorganic insulating material, for example, an alkaline metal silicate, and then overcoated with a polymer layer. Similar doubly-coated particles are disclosed in U.S. Pat. Nos. 1,850,181 and 1,789,477, both to Roseby. The Roseby particles are treated with phosphoric acid prior to molding the particles into magnetic cores. A varnish is used as a binder during the molding operation and acts as a partial insulating layer. Other doubly-coated particles which are first treated with phosphoric acid are disclosed in U.S. Pat. No. 2,783,208, Katz, and U.S. Pat. No. 3,232,352, Verweij. In both the Katz and Verweij disclosures, a thermosetting phenolic material is utilized during molding to form an insulating binder. More recently, U.S. Pat. No. 5,063,011 to Rutz et al., discloses polymer-coated iron particles where the iron particles are first treated with phosphoric acid and then coated with a polyethersulfone or a polyetherimide.
An improvement in the processing of metal-based powder particles to form core components is disclosed in U.S. Pat. No. 5,268,140 to Rutz et al. In the ""140 patent, iron-based particles are coated with a thermoplastic material and compacted under heat and pressure to form a core component. The component produced is subsequently heat treated at a temperature above the glass transition temperature of the thermoplastic material to improve the strength of the core component.
Despite the advantages of producing core components from the aforementioned insulated metal-based powder particles, in AC applications, the magnetic core components can have significant core losses at low frequencies of about 500 Hz or less. These core losses are due to coercive forces that are produced or increased during the compressing (e.g., cold working) of the insulated metal-based powder particles. The coercive force of a magnetic core component is the magnetic force needed to overcome magnetic forces that were retained when the magnetic core component was exposed to a magnetic field. In addition to increased coercive forces, the cold working of the metal-based powder particles during compression can also reduce the permeability of the magnetic core component.
One way to reduce coercive forces (resulting in core losses), and to increase the permeability of a core component, is to subject the core component to temperatures of at least about 480xc2x0 C. (hereinafter referred to as xe2x80x9chigh temperature annealingxe2x80x9d). By performing such high temperature annealing, core losses are reduced by decreasing the coercive forces of the magnetic core component. This reduction in coercive force results from a xe2x80x9crecovery processxe2x80x9d whereby metal lattices in the metal powder that are strained during compression recover their physical and mechanical properties prior to compression. High temperature annealing also has the benefit of increasing the strength of the core component without having to add additional components, such as binders. However, for such processes, the insulating material must be one that is not destroyed or decomposed upon exposure to these temperatures.
U.S. Pat. No. 4,927,473 to Ochiai et al., discloses an annealable iron-based powder composition in which the insulating layer on the particles is an inorganic compound or a metal alkoxide. For the inorganic compound, Ochiai teaches the use of materials that have an electronegativity sufficiently larger or smaller than that of iron, so that particles of the inorganic compound can be dispersed on the iron particles by electrostatic forces. However, since such an insulating layer is comprised of discrete inorganic particles attached to the iron particles, it is not xe2x80x9cfully protectivexe2x80x9d or continuous.
Thus, there is a need for an insulating material that can withstand annealing temperatures of at least about 480xc2x0 C., and that can coat the surfaces of metal-based core particles to form a substantially continuous and nonporous insulating layer surrounding the metal-based core particles. There is also a need for annealable insulated metal-based powder particles that can be compressed into core components having improved magnetic performance under AC or DC operating conditions. There is also a need for core components that have low core losses at frequencies of about 500 Hz or lower.
The present invention provides annealable insulated metal-based powder particles for forming core components, and methods of making and using the same. The annealable insulated metal-based powder particles comprise the metal-based core particles; and from about 0.001 percent by weight to about 15 percent by weight, based on the weight of the metal-based core particles, of a layer of an annealable insulating material surrounding the metal-based core particles. The annealable insulating material comprises at least one organic polymeric resin and at least one inorganic compound that is converted upon heating to a substantially continuous and nonporous insulating layer that circumferentially surrounds each of the metal-based core particles. Preferably, the inorganic compound is converted to the continuous layer at temperatures of about 480xc2x0 C. or higher.
The annealable insulated particles are prepared in accordance with the present invention by providing the annealable insulating material in a coatable form, and coating the material onto the metal-based core particles to form a layer of the insulating material surrounding the metal-based core particles.
The annealable insulated metal-based powder particles thus produced can be formed into core components in accordance with the present invention by compacting the annealable insulated particles at conventional pressures to form a core component, heating the core component to form the layer of the annealable insulating material into a substantially continuous and nonporous insulating layer that circumferentially surrounds each of the metal-based core particles, and annealing the core component at a temperature of at least about 480xc2x0 C. The core components produced are useful in both AC and DC operating conditions, and are particularly useful in low frequency AC applications of 500 Hz or less.
In a preferred embodiment of the present invention, the annealable insulated metal-based powder particles further comprise an inner layer of a preinsulating material located circumferentially between the metal-based core particles and the layer of the annealable insulating material. Preferably, this inner layer of preinsulating material is a phosphorus-iron reaction product, such as iron phosphate. This inner layer of preinsulating material further enhances the performance of the annealable insulated metal-based powder particles in magnetic core components in AC applications.